Light emitting device including tandem structure

ABSTRACT

A light emitting device comprising: a pair of electrodes; two or more light emitting elements disposed between the electrodes in a stacked arrangement, wherein a light emitting element comprises a layer comprising an emissive material; and a charge generation element disposed between adjacent light emitting elements in the stacked arrangement, the charge generation element comprising a first layer comprising an inorganic n-type semiconductor material, and a second layer comprising a hole injection material. A charge generation element is also disclosed.

This application is a continuation of International Application No.PCT/US2013/059432, filed 12 Sep. 2013, which was published in theEnglish language as International Publication No. WO 2014/088667 on 12Jun. 2014, which International Application claims priority to U.S.Provisional Patent Application No. 61/701,379, filed on 14 Sep. 2012.Each of the foregoing is hereby incorporated herein by reference in itsentirety for all purposes.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Contract No.2004*H838109*000 awarded by the Central Intelligence Agency. TheGovernment has certain rights in the invention.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the technical field of light emittingdevices and components thereof.

SUMMARY OF THE INVENTION

The present invention relates to a charge generation element and a lightemitting device including two or more light emitting elements with acharge generation element between two adjacent light emitting elements.

In accordance with one aspect of the present invention, there isprovided a light emitting device comprising: a pair of electrodes; twoor more light emitting elements disposed between the electrodes in astacked arrangement, wherein a light emitting element comprises a layercomprising an emissive material; and a charge generation elementdisposed between adjacent light emitting elements in the stackedarrangement, the charge generation element comprising a first layercomprising an inorganic n-type semiconductor material and a second layercomprising a hole injection material.

A light emitting element can include a layer comprising an emissivematerial comprising quantum dots.

A light emitting element can include a layer comprising an emissivematerial comprising an organic electroluminescent material.

A light emitting device can include one or more light emitting elementsthat include a layer comprising an emissive material comprising quantumdots and one or more light emitting elements that include an organicelectroluminescent material.

In embodiments including a light emitting element including a layercomprising an emissive material comprising quantum dots, such layercomprising quantum dots can be contiguous to the first layer of thecharge generation element.

A light emitting element included in the light emitting device can emitlight having a wavelength that is the same as or different from thatemitted by another light emitting element included in the device.

An inorganic n-type semiconductor material can inject electrons into alight emitting element adjacent the first layer of the charge generationelement.

A hole injection material can inject holes into a light emitting elementadjacent the second layer of the charge generation element.

A light emitting element can further include one or more additionalfunctional layers.

A light emitting device can further include one or more additionalfunctional layers.

In accordance with another aspect of the present invention there isprovided a charge generation element comprising a first layer comprisingan inorganic n-type semiconductor material and a second layer comprisinga hole injection material.

In the various aspects and embodiments of the inventions describedherein, a charge generation element can include an inorganic n-typesemiconductor material that can inject electrons into a light emittingelement adjacent the first layer of the charge generation element.Examples of inorganic n-type semiconductor materials comprise n-typesemiconductor materials comprising a Group II-VI compound, a Group III-Vcompound, a Group IV-VI compound, a Group I-III-VI compound, a GroupII-IV-VI compound, a Group II-IV-V compound, a Group IV element, analloy including any of the foregoing, and/or a mixture including any ofthe foregoing.

Preferably, the inorganic n-type semiconductor material comprisesnanoparticles of the inorganic n-type semiconductor material.

In the various aspects and embodiments of the inventions describedherein, a charge generation element can include a hole injection layercomprising a material that can inject holes into a light emittingelement adjacent the second layer of the charge generation element.Examples of hole injection materials comprise known organic and/orinorganic hole injection materials. Such materials are readilyidentifiable by one of ordinary skill in the relevant art.

In the various aspects and embodiments of the inventions describedherein, a charge generation element is preferably at least 50%transparent to the passage of light therethrough.

In the various aspects and embodiments of the inventions describedherein, unless otherwise expressly provided, each layer can comprise oneor more sublayers and each layer or sublayer can comprise one or morematerials.

The foregoing, and other aspects described herein, all constituteembodiments of the present invention.

It should be appreciated by those persons having ordinary skill in theart(s) to which the present invention relates that any of the featuresdescribed herein in respect of any particular aspect and/or embodimentof the present invention can be combined with one or more of any of theother features of any other aspects and/or embodiments of the presentinvention described herein, with modifications as appropriate to ensurecompatibility of the combinations. Such combinations are considered tobe part of the present invention contemplated by this disclosure.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention as claimed.

Other embodiments will be apparent to those skilled in the art fromconsideration of the description and drawings, from the claims, and frompractice of the invention disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 depicts a diagram of an example of an embodiment of the presentinvention including a stacked arrangement of two light emitting elementswith a charge generation element in between.

FIG. 2 provides a graphical representation of lumens/watt as a functionof current density for two separate single devices and for an example ofan embodiment of tandem device in accordance with the present invention.

FIG. 3 provides a graphical representation of candelas/amp as a functionof current density for two separate single devices and for an example ofan embodiment of tandem device in accordance with the present invention.

FIG. 4 provides a graphical representation of electroluminescentintensity (in arbitrary units) as a function of time for a single celldevice and for an example of an embodiment of tandem device inaccordance with the present invention.

FIG. 5 depicts a diagram of an example of light emitting device withouta tandem structure (and without a charge generation element).

The attached figures provide a simplified representation presented forpurposes of illustration only; actual structures may differ in numerousrespects, including, e.g., relative scale, etc.

For a better understanding to the present invention, together with otheradvantages and capabilities thereof, reference is made to the followingdisclosure and appended claims in connection with the above-describeddrawings.

DETAILED DESCRIPTION OF THE INVENTION

Various aspects and embodiments of the present inventions will befurther described in the following detailed description.

In accordance with one aspect of the present invention there is provideda charge generation element comprising a first layer comprising aninorganic n-type semiconductor material and a second layer comprising ahole injection material.

A charge generation element can be useful in a light emitting deviceincluding two or more light emitting structures in a stacked arrangement(also referred to herein as a tandem structure) between a pair ofelectrodes, wherein a charge generation element is disposed between twoadjacent light emitting structures. The first layer of the chargegeneration element can inject electrons into the light emittingstructure (or light emitting element) adjacent the first layer and thesecond layer can inject holes into the light emitting structure adjacentthe second layer of the charge generation element.

Charge generation element(s) can facilitate improvement in the luminanceefficiency (Cd/A) of a tandem structure light emitting device includingsame compared to a device including a single light emitting elementbetween two electrodes.

A lifetime benefit can also be obtained in a tandem structure lightemitting device including charge generation element between adjacentlight emitting structures.

Further, by stacking multiple emitting elements in one device, currentshort/leakage path can be reduced.

A charge generation element is preferably at least 50% transparent tothe passage of light, including, for example, at least 60% transparentto the passage of light, at least 70% transparent to the passage oflight, at least 80% transparent to the passage of light, at least 90%transparent to the passage of light, at least 95% up to 100% transparentto the passage of light.

Inorganic n-type semiconductor materials that can be included in a firstlayer are known.

An inorganic n-type semiconductor material can comprise n-typesemiconductor material comprising a Group II-VI compound, a Group III-Vcompound, a Group IV-VI compound, a Group I-III-VI compound, a GroupII-IV-VI compound, a Group II-IV-V compound, a Group IV element, analloy including any of the foregoing, and/or a mixture including any ofthe foregoing.

Examples of inorganic semiconductor materials include, but are notlimited to, a Group II-chalcogenide (such as a metal oxide, a metalsulfide, a metal selenide, a metal telluride), a Group III-pnictide(such as a metal nitride, a metal phosphide, a metal arsenide, or metalarsenide), or elemental semiconductor. For example, an inorganicsemiconductor material can include, without limitation, zinc oxide, atitanium oxide, a niobium oxide, an indium tin oxide, copper oxide,nickel oxide, vanadium oxide, chromium oxide, indium oxide, tin oxide,gallium oxide, manganese oxide, iron oxide, cobalt oxide, aluminumoxide, thallium oxide, silicon oxide, germanium oxide, lead oxide,zirconium oxide, molybdenum oxide, hafnium oxide, tantalum oxide,tungsten oxide, cadmium oxide, iridium oxide, rhodium oxide, rutheniumoxide, osmium oxide, zinc sulfide, zinc selenide, zinc telluride,cadmium sulfide, cadmium selenide, cadmium telluride, mercury sulfide,mercury selenide, mercury telluride, silicon carbide, diamond (carbon),silicon, germanium, aluminum nitride, aluminum phosphide, aluminumarsenide, aluminum antimonide, gallium nitride, gallium phosphide,gallium arsenide, gallium antimonide, indium nitride, indium phosphide,indium arsenide, indium antimonide, thallium nitride, thalliumphosphide, thallium arsenide, thallium antimonide, lead sulfide, leadselenide, lead telluride, iron sulfide, indium selenide, indium sulfide,indium telluride, gallium sulfide, gallium selenide, gallium telluride,tin selenide, tin telluride, tin sulfide, magnesium sulfide, magnesiumselenide, magnesium telluride, barium titanate, barium zirconate,zirconium silicate, yttria, silicon nitride, and a mixture of two ormore thereof.

An inorganic n-type semiconductor material can include an n-type dopant.

Examples of n-dopants for use in an inorganic semiconductor materialtaught herein include, but are not limited to, indium, aluminum, andgallium. As the skilled artisan will recognize other n-dopants can alsobe used.

An example of a preferred inorganic n-type semiconductor material forinclusion in a charge generation element comprises n-type zinc oxide. Incertain embodiments, zinc oxide can be mixed or blended with one or moreother inorganic materials, e.g., inorganic semiconductor materials, suchas titanium oxide.

Other examples of inorganic n-type semiconducting materials includetitanium oxide and indium phosphide.

Inorganic n-type semiconductor materials in the form of nanoparticlescan be preferred for inclusion in the first layer of the chargegeneration element.

Preferably such nanoparticles are non-light-emitting.

In certain embodiments, it may be desirable that the surfaces of thenanoparticles are not passivated. For example, it may be desirable thatthe nanoparticles not include surface ligands or other surfacepassivation treatment.

Inorganic semiconductor nanoparticles can be prepared by knowntechniques, including, but not limited to, by a colloidal solutionprocess.

Such nanoparticles can have an average particle size less than 20 nm.For example, the average particle size can be less than 10 nm, less than5 nm, less than 4 nm, less than 2 nm. In certain embodiments, thenanoparticles can have an average particle size in a range from about 2to about 6 nm. Other sizes may also be determined by the skilled art tobe useful and/or desirable. In certain embodiments, a uniform orsubstantially uniform nanoparticle size may be desirable. In certainembodiments, a nonuniform nanoparticle size may be desirable.

A layer comprising nanoparticles of an inorganic material can beprepared, for example, by forming a layer from a dispersion of thenanoparticles in a liquid and removing the liquid. The liquid can beremoved by evaporation, heat, or other technique identified by theskilled artisan. A preferred liquid is one in which the nanoparticlesare not altered so as to change the composition or size thereof in a waythat is not intended or desired.

Inorganic semiconductor materials can be deposited at a low temperature,for example, by a known method, such as a vacuum vapor depositionmethod, an ion-plating method, sputtering, inkjet printing, sol-gel,etc. For example, sputtering is typically performed by applying a highvoltage across a low-pressure gas (for example, argon) to create aplasma of electrons and gas ions in a high-energy state. Energizedplasma ions strike a target of the desired coating material, causingatoms from that target to be ejected with enough energy to travel to,and bond with, the substrate. Other techniques for depositing inorganicsemiconductor materials can also be used.

A charge generation element also includes a second layer comprising ahole injection material.

A hole injection material is a material capable of injecting holes.

Hole-injection materials are known. Examples of hole injection materialscomprise known organic and/or inorganic hole injection materials,readily identifiable by one of ordinary skill in the relevant art.

A hole-injection material can be inorganic or organic.

Examples of organic hole injection materials include, but are notlimited to, LG-101 (see, for example, paragraph [0024] of EP 1 843 411Al) and other hole injection materials available from LG Chem, LTD.Other organic hole injection materials can be used. For example, a holeinjection material can comprise a comprise a hole transport layer thathas been doped, preferably p-type doped. Examples of p-type dopantsinclude, but are not limited to, stable, acceptor-type organic molecularmaterial, which can lead to an increased hole conductivity in the dopedlayer, in comparison with a non-doped layer. In certain embodiments, adopant comprising an organic molecular material can have a highmolecular mass, such as, for example, at least 300 amu. Examples ofdopants include, without limitation, F₄-TCNQ, FeCl₃, etc. Examples ofdoped organic materials for use as a hole injection material include,but are not limited to, an evaporated hole transport materialcomprising, e.g., 4,4′,4″-tris(diphenylamino)triphenylamine (TDATA) thatis doped with tetrafluoro-tetracyano-quinodimethane (F₄-TCNQ); p-dopedphthalocyanine (e.g., zinc-phthalocyanine (ZnPc) doped with F₄-TCNQ (at,for instance, a molar doping ratio of approximately 1:30);N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′biphenyl-4,4″diamine (alpha-NPD)doped with F₄-TCNQ. See J. Blochwitz, et al., “Interface ElectronicStructure Of Organic Semiconductors With Controlled Doping Levels”,Organic Electronics 2 (2001) 97-104; R. Schmechel, 48, InternationalesWissenschaftliches Kolloquium, Technische Universtaat Ilmenau, 22-25Sep. 2003; C. Chan et al., “Contact Potential Difference Measurements OfDoped Organic Molecular Thin Films”, J. Vac. Sci. Technol. A 22(4),July/August 2004. The disclosures of the foregoing papers are herebyincorporated herein by reference in their entireties. See also, Examplesof p-type doped inorganic hole transport materials are described in U.S.Patent Application No. 60/653,094 entitled “Light Emitting DeviceIncluding Semiconductor Nanocrystals, filed 16 Feb. 2005, which ishereby incorporated herein by reference in its entirety. Examples ofp-type doped organic hole transport materials are described in U.S.Provisional Patent Application No. 60/795,420 of Beatty et al, for“Device Including Semiconductor Nanocrystals And A Layer Including ADoped Organic Material And Methods”, filed 27 Apr. 2006, which is herebyincorporated herein by reference in its entirety.

Examples of inorganic hole injection materials are known and can bereadily identified by the person of ordinary skill in the art.Molybdenum oxide is an example of an inorganic hole injection materialsthat may be preferred.

In accordance with another aspect of the present invention, there isprovided a light emitting device comprising: a pair of electrodes; twoor more light emitting elements disposed between the electrodes in astacked arrangement, wherein a light emitting element comprises a layercomprising an emissive material; and a charge generation elementdisposed between adjacent light emitting elements in the stackedarrangement, the charge generation element comprising a first layercomprising an inorganic n-type semiconductor material and a second layercomprising a hole injection material.

FIG. 1 provides a cross-sectional view of a schematic representation ofan example of a light-emitting device according to one embodiment of thepresent invention. While the depicted example has an inverted structure(e.g., the negative electrode or cathode is placed in contact with thesubstrate), the present invention also applies to a light emittingdevice having a non-inverted structure (e.g., the positive electrode oranode is place in contact with the substrate). Referring to FIG. 1, thelight-emitting device 100 includes (from bottom to top) a firstelectrode (e.g., a cathode)/substrate combination 1, a first lightemitting element comprising a layer comprising a material capable oftransporting charge (depicted as a material capable of transportingelectrons, which is also referred to herein as a “electron transportmaterial”) 6, an emissive layer comprising an emissive material (e.g.,quantum dots) 7, a layer comprising a material capable of transportingcharge (depicted as a material capable of transporting holes, which isalso referred to herein as a “hole transport material”) 8, a chargegeneration element comprising a second layer comprising a hole injectionmaterial 11 (adjacent the hole transport layer of the first lightemitting element) and a first layer comprising an inorganic n-typesemiconductor material 12, a second light emitting element comprising alayer comprising an emissive material (e.g., quantum dots) 16, and alayer comprising a hole transport material 17.

The depicted light-emitting device 100 also includes a hole injectionlayer 18 between the uppermost light emitting element and the secondelectrode (e.g., an anode) 19. The inclusion of such hole injectionlayer is optional, but may be desirable depending upon the electrodematerial and the particular material included in the adjacent layer ofthe uppermost light emitting element.

A light emitting element includes at least a layer comprising anemissive material. A light emitting element can further include one ormore additional layers.

In certain embodiments, the light emitting device can further includemore than two light emitting elements in the stacked arrangement with acharge generation element between any two light adjacent emittingelements.

A substrate can be opaque or transparent. A transparent substrate can beused, for example, in the manufacture of a transparent light emittingdevice, as shown in the FIG. 1. See, for example, Bulovic, V. et al.,Nature 1996, 380, 29; and Gu, G. et al., Appl. Phys. Lett. 1996, 68,2606-2608, each of which is incorporated by reference in its entirety.

Transparent substrates including patterned ITO are commerciallyavailable and can be used in making a device according to the presentinvention as shown in FIG. 1.

A substrate can be rigid or flexible. For example, a substrate can beplastic, metal, semiconductor wafer, or glass. A substrate can be asubstrate commonly used in the art. Preferably a substrate has a smoothsurface. A substrate surface free of defects is particularly desirable.

One electrode can be, for example, an anode comprising a high workfunction (e.g., greater than 4.0 eV) hole-injecting conductor, such asan indium tin oxide (ITO) layer. Other anode materials include otherhigh work function hole-injection conductors including, but not limitedto, for example, tungsten, nickel, cobalt, platinum, palladium and theiralloys, gallium indium tin oxide, zinc indium tin oxide, titaniumnitride, polyaniline, or other high work function hole-injectionconducting polymers. An electrode can be light transmissive ortransparent. In addition to ITO, examples of other light-transmissiveelectrode materials include conducting polymers, and other metal oxides,low or high work function metals, conducting epoxy resins, or carbonnanotubes/polymer blends or hybrids that are at least partially lighttransmissive. An example of a conducting polymer that can be used as anelectrode material is poly(ethlyendioxythiophene), sold by Bayer AGunder the trade mark PEDOT. Other molecularly altered poly(thiophenes)are also conducting and could be used, as well as emaraldine salt formof polyaniline.

The other electrode can be, for example, a cathode comprising a low workfunction (e.g., less than 4.0 eV), electron-injecting, metal, such asAl, Ba, Yb, Ca, a lithium-aluminum alloy (Li:Al), a magnesium-silveralloy (Mg:Ag), or lithium fluoride-aluminum (LiF:Al). Other examples ofcathode materials include silver, gold, ITO, etc. An electrode, such asMg:Ag, can optionally be covered with an opaque protective metal layer,for example, a layer of Ag for protecting the cathode layer fromatmospheric oxidation, or a relatively thin layer of substantiallytransparent ITO.

An electrode can be sandwiched, sputtered, or evaporated onto theexposed surface of the solid layer.

One or both of the electrodes can be patterned.

The electrodes of the device can be connected to a voltage source byelectrically conductive pathways.

In certain aspects and embodiments of the inventions described herein,the substrate, electrode (e.g., anode or cathode) materials and othermaterials included in a device are selected based on light transparencycharacteristics. For example, a device comprising a light-emittingdevice, such selection can enable preparation of a device that emitslight from the top surface thereof. A top emitting device can beadvantageous for constructing an active matrix device (e.g., a display).In another example, such selection can enable preparation of a devicethat emits light from the bottom surface thereof. In yet anotherexample, such selection can enable preparation of a device that istransparent on both sides (e.g., fully transparent). Preferably anemitting side is at least 50% transparent. As the skilled artisan willappreciate, higher transparency can be more preferable.

In a light-emitting device, electroluminescence can be produced by theemissive material included in the device when a voltage of properpolarity is applied across the device.

As shown in FIG. 1, the first electrode 1 can be a cathode. As discussedabove, a cathode can comprise a material that can easily injectelectrons. Examples include, but are not limited to, ITO, aluminum,silver, gold, etc. Other suitable cathode materials are known and can bereadily ascertained by the skilled artisan. The electrode material canbe deposited using any suitable technique. In certain embodiments, theanode can be patterned. Alternatively, the first electrode can be ananode, with the other electrode being the cathode.

As described above, a light emitting element comprises a layercomprising an emissive material. An emissive layer can be patterned orunpatterned.

A layer comprising an emissive material can comprise one or more layersof the same or different emissive material(s).

In certain embodiments, the emissive layer can have a thickness in arange from about 1 nm to about 20 nm. In certain embodiments, theemissive layer can have a thickness in a range from about 1 nm to about10 nm. In certain embodiments, the emissive layer can have a thicknessin a range from about 3 nm to about 6 about nm. In certain embodiments,the emissive layer can have a thickness of about 4 nm.

An emissive material can comprise quantum dots (which quantum dots mayfurther include ligands attached thereto). Quantum dots and ligands arefurther discussed below.

An emissive material can comprise one or more different types of quantumdots, wherein each type can be selected to emit light having apredetermined wavelength. In certain embodiments, quantum dot types canbe different based on, for example, factors such composition, structureand/or size of the quantum dot.

Emissive materials that emit light having different wavelengths can beincluded in a single layer and/or in separate layers.

Quantum dots can be selected to emit at any predetermined wavelengthacross the electromagnetic spectrum.

Different types of quantum dots that have emissions at the same and/ordifferent wavelengths can be utilized.

In certain embodiments, quantum dots can be capable of emitting visiblelight.

In certain embodiments, quantum dots can be capable of emitting infraredlight.

The color of the light output of an emissive material comprising quantumdots can be controlled by the selection of the composition, structure,and size of the quantum dots included in the emissive material.

An emissive material can comprise an organic electroluminescentmaterial. Examples of organic electroluminescent materials include thoseused or suitable for use as the emissive material in an organic lightemitting device (OLED). Such organic electroluminescent materials can bereadily ascertained by those of ordinary skill in the relevant art.

An emissive material can comprise a combination of quantum dots and oneor more organic electroluminescent materials in a mixed and/or layeredarrangement.

A light emitting device in accordance with the present invention caninclude one or more light emitting elements including an emissivematerial comprising quantum dots and one or more light emitting elementsincluding an emissive material comprising an organic electroluminescentmaterial.

A light emitting device in accordance with the present invention caninclude two or more light emitting elements wherein each of the lightemitting elements includes an emissive material comprising quantum dots.

A light emitting device in accordance with the present invention caninclude two or more light emitting elements wherein each of the lightemitting elements includes an emissive material comprising an organicelectroluminescent material.

Preferably, at least one of the light emitting elements includes anemissive material comprising quantum dots.

A light emitting element can further include one or more layersincluding a charge transport material (e.g., a layer comprising anelectron transport material (also referred to herein as an electrontransport layer) and/or a layer comprising a hole transport material(also referred to herein as a hole transport material)).

For example, as shown in FIG. 1, the first light emitting element 5includes a layer including an emissive material sandwiched between anelectron transport layer and a hole transport layer (preferablyincluding a hole transport material comprising a spiro-compound); andthe second light emitting element 15 includes a layer comprising theemissive material and a hole transport layer. (If a hole transport layerin a light emitting element adjacent the hole generation electrodecomprises a material with both hole injection and hoe transportcapability it can serve the function of both a hole transport layer anda hole injection layer in a single layer.)

A light emitting element can further include additional features (e.g.,additional layers).

Advantageously, the inorganic n-type semiconductor material included ina charge generation element in accordance with the present invention canalso serve as an electron transport layer for the light emitting elementcontiguous thereto.

Hole transport and electron transport materials can be collectivelyreferred to as charge transport materials. Either or both of thesematerials can comprise organic or inorganic materials. Examples ofinorganic materials include, for example, inorganic semiconductors. Suchinorganic material can be amorphous or polycrystalline. An organiccharge transport material can be polymeric or non-polymeric.

Organic hole transport materials are known. An example of an organicmaterial that can be included in a hole transport layer includes anorganic chromophore. The organic chromophore can include a phenyl amine,such as, for example,N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine(TPD). Other hole transport layer can include(N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-spiro (spiro-TPD),4-4′-N,N′-dicarbazolyl-biphenyl (CBP),4,4-.bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPD), etc., apolyaniline, a polypyrrole, a poly(phenylene vinylene), copperphthalocyanine, an aromatic tertiary amine or polynuclear aromatictertiary amine, a 4,4′-bis(p-carbazolyl)-1,1′-biphenyl compound,N,N,N′,N′-tetraarylbenzidine, poly(3,4-ethylenedioxythiophene)(PEDOT)/polystyrene para-sulfonate (PSS) derivatives,poly-N-vinylcarbazole derivatives, polyphenylenevinylene derivatives,polyparaphenylene derivatives, polymethacrylate derivatives,poly(9,9-octylfluorene) derivatives, poly(spiro-fluorene) derivatives,N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB),tris(3-methylphenylphenylamino)-triphenylamine (m-MTDATA), andpoly(9,9′-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine (TFB), andspiro-NPB.

In certain preferred embodiments, a hole transport layer comprises anorganic small molecule material, a polymer, a spiro-compound (e.g.,spiro-NPB), etc.

In certain embodiments of the inventions described herein, a holetransport layer can comprise an inorganic material. Inorganic holetransport materials are known. Examples of inorganic materials include,for example, inorganic semiconductor materials capable of transportingholes. The inorganic material can be amorphous or polycrystalline.Examples of such inorganic materials and other information related tofabrication of inorganic hole transport materials that may be helpfulare disclosed in International Application No. PCT/US2006/005184, filed15 Feb. 2006, for “Light Emitting Device Including SemiconductorNanocrystals, which published as WO 2006/088877 on 26 Aug. 2006, thedisclosure of which is hereby incorporated herein by reference in itsentirety.

Organic electron transport materials are known. An example of a typicalorganic material that can be included in an electron transport layerincludes a molecular matrix. The molecular matrix can be non-polymeric.The molecular matrix can include a small molecule, for example, a metalcomplex. The metal complex of 8-hydoryquinoline can be an aluminum,gallium, indium, zinc or magnesium complex, for example, aluminumtris(8-hydroxyquinoline) (Alq₃). In certain embodiments, the electrontransport material can comprise LT-N820 or LT-N821(1,3-Bis[2-(2,2′-bypyridine-6-yl)-1,3,4-oxadiazo-5-yl]benzene (alsoabbreviated as Bpy-OXD), available from Luminescent Technologies,Taiwan. For additional information relating to Bpy-OXD, see M. Ichikawaet al., J. Mater. Chem., 2006, 16, 221-25, the disclosure of which ishereby incorporated herein by reference in its entirety. Other classesof materials in the electron transport layer can include metalthioxinoid compounds, oxadiazole metal chelates, triazoles,sexithiophenes derivatives, pyrazine, and styrylanthracene derivatives.An electron transport layer comprising an organic material may beintrinsic (undoped) or doped. Doping may be used to enhanceconductivity. See, for example, U.S. Provisional Patent Application No.60/795,420 of Beatty et al, for “Device Including SemiconductorNanocrystals And A Layer Including A Doped Organic Material AndMethods”, filed 27 Apr. 2006, which is hereby incorporated herein byreference in its entirety.

An electron transport material can comprise an inorganic material.Inorganic electron transport materials are known. Examples of inorganicmaterials include, for example, inorganic semiconductor materialscapable of transporting holes. The inorganic material can be amorphousor polycrystalline. Examples of such inorganic materials and otherinformation related to fabrication of inorganic hole transport materialsthat may be helpful are disclosed in International Application No.PCT/US2006/005184, filed 15 Feb. 2006, for “Light Emitting DeviceIncluding Semiconductor Nanocrystals, which published as WO 2006/088877on 26 Aug. 2006, the disclosure of which is hereby incorporated hereinby reference in its entirety.

A material having electron transport and electron injection capabilitiescan be included in a light emitting element as an electron transportmaterial. A preferred example of such a material includes zinc oxide.Additional information about materials having electron transport andelectron injection capabilities is described in U.S. Patent ApplicationPublication No. 20110140075, published Jun. 16, 2011, of Zhou, et al.,entitled “Light-Emitting Device Including Quantum Dots”, which is herebyincorporated herein by reference.

Charge transport materials comprising, for example, an inorganicmaterial such as an inorganic semiconductor material, can be depositedat a low temperature, for example, by a known method, such as a vacuumvapor deposition method, an ion-plating method, sputtering, inkjetprinting, sol-gel, etc. Other techniques can also be used.

Organic charge transport materials may be deposited by known methodssuch as a vacuum vapor deposition method, a sputtering method, adip-coating method, a spin-coating method, a casting method, abar-coating method, a roll-coating method, and other film depositionmethods. Preferably, organic layers are deposited under ultra-highvacuum (e.g., ≦10⁻⁸ torr), high vacuum (e.g., from about 10⁻⁸ torr toabout 10⁻⁵ torr), or low vacuum conditions (e.g., from about 10⁻⁵ torrto about 10⁻³ torr). Other techniques can also be used.

Charge transport materials comprising organic materials and otherinformation related to fabrication of organic charge transport layersthat may be helpful are disclosed in U.S. patent application Ser. No.11/253,612 for “Method And System For Transferring A PatternedMaterial”, filed 21 Oct. 2005, and Ser. No. 11/253,595 for “LightEmitting Device Including Semiconductor Nanocrystals”, filed 21 Oct.2005, each of which is hereby incorporated herein by reference in itsentirety.

A charge transport material is preferably included in a light emittingelement as a layer. In certain embodiments, the layer can have athickness in a range from about 10 nm to about 500 nm.

As discussed above, a light emitting device can further include a layercomprising a hole-injection material between the hole generatingelectrode and an adjacent hole transport layer of a light emittingelement. Hole injection materials are discussed elsewhere herein.

In certain embodiments, a layer comprising a spacer material (not shown)can be included between the emissive material and an adjacent layer. Alayer comprising a spacer material can promote better electricalinterface between the emissive layer and the adjacent layer. A spacermaterial may comprise an organic material or an inorganic material. Incertain embodiments, a spacer material comprises parylene. Preferably,the spacer material comprises an ambipolar material. More preferably, itis non-quenching. In certain embodiments, for example, a spacer materialbetween the emissive layer and a hole transport layer can comprise anambipolar host or hole transport material, or nanoparticles such asnickel oxide, and other metal oxides.

A light emitting element can further optionally include one or moreinterfacial layers as described, for example, in InternationalApplication No. PCT/US2010/051867 of QD Vision, Inc., et al. entitled“Device Including Quantum Dots”, which published as WO 2011/044391 on 14Apr. 2011, which is hereby incorporated herein by reference in itsentirety.

A charge generation element 10 comprises a first layer comprising aninorganic n-type semiconductor material and a second layer comprising ahole injection material.

Inorganic n-type semiconductor materials and hole injection materialsuseful in a charge generation element are discussed above.

Although FIG. 1 depicts an example of an embodiment of a light emittingdevice including two light emitting elements, as mentioned above, alight emitting device in accordance with the present invention can alsocomprise three or more light emitting elements that are stacked. When acharge generation element is provided between the pair of electrodes soas to partition adjacent light emitting elements in the stackedarrangement in the manner depicted in FIG. 1, the light emitting deviceis similarly expected to have an improved electroluminescent lifetimeand luminance while keeping a low current density.

When the light emitting elements included in a light emitting device arecapable of emitting light having different colors from each other, acomposite light emission of a desired color can be obtained from thelight emitting device. For example, in a light emitting device havingtwo light emitting elements, the emission colors of the first lightemitting element and the second light emitting element can be selectedto emit a desired composite light color from the device. For example,composite white light can be obtained from a light emitting deviceincluding a blue light emitting element and a yellow light emittingelement. Composite white light can also be obtained, for example, from alight emitting device including three light emitting elements (e.g.,red, blue, and green). Other combinations of light emitting elementsthat emit light having preselected color light can be arranged in astacked arrangement to provide other desired composite color output fromthe light emitting device. Selection of the colors of light to beemitted from the light emitting elements included in the tandemstructure to obtain the desired composite color light from the device iswithin the skill of a person of ordinary skill in the relevant art.

As discussed above, a light emitting element is not limited to theexample of the embodiment depicted in FIG. 1.

A light emitting device described herein can further include apassivation or other protective layer or coating that can be used toprotect the device from the environment. For example, a protective glasslayer can be included to encapsulate the device. Optionally a desiccantor other moisture absorptive material can be included in the devicebefore it is sealed, e.g., with an epoxy, such as a UV curable epoxy.Other desiccants or moisture absorptive materials can be used.

A quantum dot is a nanometer sized particle that can have opticalproperties arising from quantum confinement. The particularcomposition(s), structure, and/or size of a quantum dot can be selectedto achieve the desired wavelength of light to be emitted from thequantum dot upon stimulation with a particular excitation source. Inessence, quantum dots may be tuned to emit light across the visiblespectrum by changing their size. See C. B. Murray, C. R. Kagan, and M.G. Bawendi, Annual Review of Material Sci., 2000, 30: 545-610 herebyincorporated by reference in its entirety.

A quantum dot can have an average particle size in a range from about 1to about 1000 nanometers (nm), and preferably in a range from about 1 toabout 100 nm. In certain embodiments, quantum dots have an averageparticle size in a range from about 1 to about 20 nm (e.g., such asabout 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 nm).In certain embodiments, quantum dots have an average particle size in arange from about 1 to about 10 nm. Quantum dots can have an averagediameter less than about 150 Angstroms ({acute over (Å)}). In certainembodiments, quantum dots having an average diameter in a range fromabout 12 to about 150 {acute over (Å)} can be particularly desirable.However, depending upon the composition, structure, and desired emissionwavelength of the quantum dot, the average diameter may be outside ofthese ranges.

For convenience, the size of quantum dots can be described in terms of a“diameter”. In the case of spherically shaped quantum dots, diameter isused as is commonly understood. For non-spherical quantum dots, the termdiameter can typically refer to a radius of revolution (e.g., a smallestradius of revolution) in which the entire non-spherical quantum dotwould fit.

Preferably, a quantum dot comprises a semiconductor nanocrystal. Incertain embodiments, a semiconductor nanocrystal has an average particlesize in a range from about 1 to about 20 nm, and preferably from about 1to about 10 nm. However, depending upon the composition, structure, anddesired emission wavelength of the quantum dot, the average diameter maybe outside of these ranges.

A quantum dot can comprise one or more semiconductor materials.

In certain preferred embodiments, the quantum dots comprise crystallineinorganic semiconductor material (also referred to as semiconductornanocrystals). Examples of preferred inorganic semiconductor materialsinclude, but are not limited to, Group II-VI compound semiconductornanocrystals, such as CdS, CdSe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, andother binary, ternary, and quaternary II-VI compositions; Group III-Vcompound semiconductor nanocrystals, such as GaP, GaAs, InP and InAs;PbS; PbSe; PbTe, and other binary, ternary, and quaternary III-Vcompositions. Other non-limiting examples of inorganic semiconductormaterials include Group II-VI compounds, Group III-V compounds, GroupIV-VI compounds, Group I-III-VI compounds, Group II-IV-VI compounds,Group II-IV-V compounds, Group IV elements, an alloy including any ofthe foregoing, and/or a mixture including any of the foregoing.

A quantum dot can comprise a core comprising one or more semiconductormaterials and a shell comprising one or more semiconductor materials,wherein the shell is disposed over at least a portion, and preferablyall, of the outer surface of the core. A quantum dot including a coreand shell is also referred to as a “core/shell” structure.

In a core/shell quantum dot, the shell or overcoating may comprise oneor more layers. The overcoating can comprise at least one semiconductormaterial which is the same as or different from the composition of thecore. Preferably, the overcoating has a thickness from about one toabout ten monolayers. An overcoating can also have a thickness greaterthan ten monolayers. In certain embodiments, more than one overcoatingcan be included on a core.

In certain embodiments, more than one shell or overcoating can beincluded in a quantum dot.

In certain embodiments, the shell can be chosen so as to have an atomicspacing close to that of the “core” substrate. In certain otherembodiments, the shell and core materials can have the same crystalstructure.

Quantum dots can also have various shapes, including, but not limitedto, sphere, rod, disk, other shapes, and mixtures of various shapedparticles.

Preferably, the quantum dots include one or more ligands attached to thesurface thereof. In certain embodiments, a ligand can include an alkyl(e.g., C₁-C₂₀) species. In certain embodiments, an alkyl species can bestraight-chain, branched, or cyclic. In certain embodiments, an alkylspecies can be substituted or unsubstituted. In certain embodiments, analkyl species can include a hetero-atom in the chain or cyclic species.In certain embodiments, a ligand can include an aromatic species. Incertain embodiments, an aromatic species can be substituted orunsubstituted. In certain embodiments, an aromatic species can include ahetero-atom. Additional information concerning ligands is providedherein and in various of the below-listed documents which areincorporated herein by reference.

By controlling the structure, shape and size of quantum dots duringpreparation, energy levels over a very broad range of wavelengths can beobtained while the properties of the bulky materials are varied. Quantumdots (including but not limited to semiconductor nanocrystals) can beprepared by known techniques. Preferably they are prepared by a wetchemistry technique wherein a precursor material is added to acoordinating or non-coordinating solvent (typically organic) andnanocrystals are grown so as to have an intended size. According to thewet chemistry technique, when a coordinating solvent is used, as thequantum dots are grown, the organic solvent is naturally coordinated tothe surface of the quantum dots, acting as a dispersant. Accordingly,the organic solvent allows the quantum dots to grow to thenanometer-scale level. The wet chemistry technique has an advantage inthat quantum dots of a variety of sizes can be uniformly prepared byappropriately controlling the concentration of precursors used, the kindof organic solvents, and preparation temperature and time, etc.

The emission from a quantum dot capable of emitting light (e.g., asemiconductor nanocrystal) can be a narrow Gaussian emission band thatcan be tuned through the complete wavelength range of the ultraviolet,visible, or infra-red regions of the spectrum by varying the size of thequantum dot, the composition of the quantum dot, or both. For example, asemiconductor nanocrystal comprising CdSe can be tuned in the visibleregion; a semiconductor nanocrystal comprising InAs can be tuned in theinfra-red region. The narrow size distribution of a population ofquantum dots capable of emitting light (e.g., semiconductornanocrystals) can result in emission of light in a narrow spectralrange. The population can be monodisperse preferably exhibits less thana 15% rms (root-mean-square) deviation in diameter of such quantum dots,more preferably less than 10%, most preferably less than 5%. Spectralemissions in a narrow range of no greater than about 75 nm, no greaterthan about 60 nm, no greater than about 40 nm, and no greater than about30 nm full width at half max (FWHM) for such quantum dots that emit inthe visible can be observed. IR-emitting quantum dots can have a FWHM ofno greater than 150 nm, or no greater than 100 nm. Expressed in terms ofthe energy of the emission, the emission can have a FWHM of no greaterthan 0.05 eV, or no greater than 0.03 eV. The breadth of the emissiondecreases as the dispersity of the light-emitting quantum dot diametersdecreases.

For example, semiconductor nanocrystals can have high emission quantumefficiencies such as greater than 10%, 20%, 30%, 40%, 50%, 60%, 70%,80%, or 90%.

The narrow FWHM of semiconductor nanocrystals can result in saturatedcolor emission. The broadly tunable, saturated color emission over theentire visible spectrum of a single material system is unmatched by anyclass of organic chromophores (see, for example, Dabbousi et al., J.Phys. Chem. 101, 9463 (1997), which is incorporated by reference in itsentirety). A monodisperse population of semiconductor nanocrystals willemit light spanning a narrow range of wavelengths. A pattern includingmore than one size of semiconductor nanocrystal can emit light in morethan one narrow range of wavelengths. The color of emitted lightperceived by a viewer can be controlled by selecting appropriatecombinations of semiconductor nanocrystal sizes and materials. Thedegeneracy of the band edge energy levels of semiconductor nanocrystalsfacilitates capture and radiative recombination of all possibleexcitons.

Transmission electron microscopy (TEM) can provide information about thesize, shape, and distribution of the semiconductor nanocrystalpopulation. Powder X-ray diffraction (XRD) patterns can provide the mostcomplete information regarding the type and quality of the crystalstructure of the semiconductor nanocrystals. Estimates of size are alsopossible since particle diameter is inversely related, via the X-raycoherence length, to the peak width. For example, the diameter of thesemiconductor nanocrystal can be measured directly by transmissionelectron microscopy or estimated from X-ray diffraction data using, forexample, the Scherrer equation. It also can be estimated from the UV/Visabsorption spectrum.

An emissive material can be deposited by spin-casting, screen-printing,inkjet printing, gravure printing, roll coating, drop-casting,Langmuir-Blodgett techniques, contact printing or other techniques knownor readily identified by one skilled in the relevant art.

Preferably, a light-emitting device including an emissive materialcomprising a plurality of semiconductor nanocrystals is processed in acontrolled (oxygen-free and moisture-free) environment, preventing thequenching of luminescent efficiency during the fabrication process.

The present invention will be further clarified by the followingexamples, which are intended to be exemplary of the present invention.

Example A. General Considerations

Unless stated otherwise, all synthetic manipulations were carried outusing standard Schlenk techniques under a nitrogen atmosphere, or in anM-Braun glovebox under an atmosphere of purified nitrogen. Zinc acetatedihydrate (>98%; Aldrich), 2-methoxyethanol (>99%, Aldrich),tetramethylammonium hydroxide (TMAH, >97%, Aldrich), 2-ethanolamine(>98%, Aldrich), trioctylphosphine (TOP, >97%, Aldrich),trioctylphosphine oxide (TOPO, >99%, Aldrich), octadecylphosphonic acid(ODPA, PCI Synthesis), dimethylcadmium (CdMe₂, 97%, Strem),hexamethyldisilathiane (TMS₂S, Synthesis grade, Aldrich), and decylamine(98%, Aldrich) were used as received. Toluene, hexanes, 2-isopropanol,and methanol were purchased from Aldrich in 18-L Pure-Pac™ solventdelivery kegs and sparged vigorously with nitrogen prior to use. CdMe₂and TMS₂S, due to their nature, should be handled with care.

B. Preparation of Inorganic n-Type ZnO Semiconducting Nanoparticles

The following procedure was done on the bench top under air. ZnOnanoparticles were synthesized by a modification of a known procedure.Zinc acetate dihydrate (3.00 g, 13.7 mmol) and 2-methoxyethanol (200 mL)were added to a flask equipped with a magnetic stirbar. In a separatevial, tetramethylammonium hydroxide (TMAH) (4.5 g, 25 mmol) and2-methoxyethanol (20 mL) were combined. Both solutions were agitatedvigorously until the reagents were dissolved. Under constant stirring,the TMAH solution was slowly added to the zinc acetate solution over 10min. The solution was allowed to stir for an additional 1 min anddemonstrated a pale blue-green emission when excited with UV light.2-Ethanolamine (4 mL) was added to stabilize the particles.

The ZnO nanoparticles were purified by adding toluene (440 mL) andhexanes (220 mL) to the above mixture. The flocculates were isolated bycentrifugation (3500 rpm; 1 min) and decanting the colorlesssupernatant. The colorless powder was redispersed in a mixture of2-isopropanol (44 mL) and methanol (11 mL) and filtered through asyringe filter (0.2 μm). The dispersion was stored at −30° C.

C. Preparation of Quantum Dots (QDs) Capable of Emitting Blue LightCdZnS/ZnS Core/Shell Preparation.

0.768 g CdO (99.9%, Aldrich) and 4.878 g ZnO (99.9%, Aldrich) wereweighed into a three necked flask equipped with a condenser. To this wasadded: 43.2 mL tech grade oleic acid (Aldrich, vacuum-transferred priorto use) and 20 mL of tech grade 1-octadecene (ODE) (Aldrich). Thecontent of the flask was degassed at 100° C. for 1 h in vacuo (10millitorr).

Separately 0.288 g of sulfur (99.9%, Strem) was dissolved in 20 mL oftech grade ODE in a three necked flask by stirring and heating to 80° C.The contents of the flask were degassed at 80° C. for 1 h in vacuo (10millitorr).

The contents of the flask containing Cd and Zn was heated to 300° C.under nitrogen for 1 h. This formed a milky suspension. The flask washeated to 310° C. and the contents of the flask containing the S/ODEwere swiftly injected into the flask containing Cd and Zn. Thetemperature fell to 275° C. and climbed back to 310° C. in ˜3 min. Theflask containing the cores was heated for a total of 10 min.

The ZnS shell was grown in situ by swiftly injecting 18.0 mL oftri-n-butylphosphine sulfide (2.66 M, prepared by adding 6.8 g of sulfurto a stirred solution of 80 mL of tri-n-butylphosphine in a glove boxover 2 h) at 310° C. This was heated for an additional 33 min. Thecontents of the flask were transferred to a degassed flask undernitrogen and transferred to an inert atmosphere box for furtherpurification

The cores including a ZnS shell were purified by precipitation asfollows. The solution was divided into 12 parts and each part was addedto a separate centrifuge tube. Butanol (ca. 25 mL) was added to eachtube. Each tube was then centrifuged for 8 min, 4000 rpm. Aftercentrifuging, the supernatant liquid was poured off, retaining the solidin each centrifuge tube. A total of ˜15 mL of hexanes was used todisperse the solid. This was divided in half and added to separatecentrifuge tubes. A total of 50 mL of butanol was added to the hexanedispersion. After mixing, each tube was centrifuged again. Thesupernatant liquid was decanted and the yellow solid was dispersed in atotal 20 mL of hexanes. The dispersion was filtered through a 0.2 micronfilter. (The overcoated cores are in the filtrate.)

Characterization of Above Prepared CdZnS/ZnS Core/Shell Nanocrystals

Maximum peak emission: 450 nm

FWHM 20 nm

Photoluminescence quantum efficiency ˜75%

Additional information relating to preparation of quantum dots can befound in X. Zhong, Y. Feng, W. Knoll, and M. Han, “AlloyedZn_(x)Cd_(1-x)S Nanocrystals with Highly Narrow Luminescence SpectralWidth,” Journal of the American Chemical Society, vol. 125, no. 44, pp.13559-13563, November 2003, W. K. Bae, M. K. Nam, K. Char, and S. Lee,“Gram-Scale One-Pot Synthesis of Highly Luminescent Blue EmittingCd_(1-x)Zn_(x)S/ZnS Nanocrystals,” Chemistry of Materials, vol. 20, no.16, pp. 5307-5313, August 2008, and U.S. Published Patent ApplicationNo. 2010/0044635, of Breen, et al., published 25 Feb. 2012, entitled“Blue Emitting Semiconductor Nanocrystals and Compositions And DevicesIncluding Same”, each of the foregoing being hereby incorporated hereinby reference in its entirety.

D. QLED Fabrication Procedures

(1) FIG. 5 A shows an example of an inverted structure where the cathodeis an ITO layer, followed by a solution-processed inorganic metal oxide(ZnO, 35 nm) as the electron-injection/transport layer followed by asolution-cast light-emitting quantum dot (QD) layer (e.g., with athickness in a range of about 10 to about 15 nm). Next, thehole-transport (spiro-2NPB, e.g., with a thickness of about 50 nm) andhole injection layers (LG101, with a thickness of about 10 nm) are vapordeposited followed by the aluminum (Al) anode contact.

(2) FIG. 1 shows an example of an embodiment of a tandem blue lightemitting device including two light emitting elements, each including anemissive material comprising quantum dots, and including a chargegeneration element between the two light emitting elements (a lightemitting element may also be referred to as a cell). The devicefabrication procedure is described as following: ZnO nanoparticles(prepared generally as described in this Example) is spun cast onto ITOas an electron injection layer and electron transport layer (e.g., witha thickness of about 35 nm), followed by forming a blue QD layer havinga thickness in a range of about 10 to about 15 nm (as generallydescribed above). Next, a layer comprising a hole-transport material(spiro-2NPB, with a thickness of about 50 nm) is vapor deposited abovethe QD layer. The hole injection layer of the charge generation element,comprising LG101, with a thickness of about 10 nm is next deposited overthe hole transport layer by vapor deposition. A layer comprisinginorganic n-type semiconductor material comprising ZnO nanoparticles isnext formed on the hole injection layer as the other part of the chargegeneration element; the ZnO nanoparticle layer being spun cast on top ofLG101. Next, a layer with a thickness in a range of about 10 to about 15nm comprising blue QDs (prepared generally as described above) is spuncast on the LG101 layer, then capped by 50 nm of Spiro-2NPB and 10 nm ofLG101. Finally, 150 nm of Al was vapor deposited on top of organic layeras anode. All devices are then sealed in a nitrogen atmosphere using aUV-curable epoxy.

FIGS. 2-4 show the results for two devices prepared generally describedin Section D above: the curve labeled “NP—ZnO Single Blue QLED”corresponding to a device prepared generally as described in paragraphD(1) and the curve labeled “NP—ZnO Tandem Blue QLED” corresponding to adevice prepared generally as described in paragraph D(2). The othercurve labeled “SG ZnO Single Blue QLED” corresponds to a single deviceincluding a structure similar to that of the device of paragraph D(1)but including a ZnO layer prepared by a sol-gel process instead of ZnOnanoparticles.

The data in FIG. 2 indicates that a tandem device in accordance with theinvention can have similar power luminous efficiency to a single celldevice.

The data in FIG. 3 indicates that a device in accordance with theinvention can have improved luminance efficiency (Cd/A) compared to asingle cell device.

The data in FIG. 4 indicates that a device in accordance with theinvention can have improved electroluminescent lifetime.

Such tandem devices, however, can also have higher operational voltagethan a single cell device and may experience a higher turn-on voltagethan a single cell device.

Other materials, techniques, methods, applications, and information thatmay be useful with the present invention are described in: InternationalApplication No. PCT/US2007/013152, filed 4 Jun. 2007, of Coe-Sullivan,et al., entitled “Light-Emitting Devices And Displays With ImprovedPerformance”; International Application No. PCT/US2010/056397 of Kazlas,et al., filed 11 Nov. 2010, entitled “Device Including Quantum Dots”,International Application No. PCT/US2008/013504, filed 8 Dec. 2008,entitled “Flexible Devices Including Semiconductor Nanocrystals, Arrays,and Methods”, of Kazlas, et al., which published as WO2009/099425 on 13Aug. 2009, International Application No. PCT/US2007/008873, filed 9 Apr.2007, of Coe-Sullivan et al., entitled “Composition Including Material,Methods Of Depositing Material, Articles Including Same And Systems ForDepositing Material”, International Application No. PCT/US2008/010651,filed 12 Sep. 2008, of Breen, et al., entitled “FunctionalizedNanoparticles And Method”, International Application No.PCT/US2009/004345, filed 28 Jul. 2009 of Breen et al., entitled“Nanoparticle Including Multi-Functional Ligand And Method”, U.S.application Ser. No. 12/283,609, filed 12 Sep. 2008 of Coe-Sullivan, etal., entitled “Compositions, Optical Component, System Including AnOptical Component, Devices, And Other Products”, and InternationalApplication No. PCT/US2009/002789 of Coe-Sullivan et al, filed 6 May2009, entitled “Solid State Lighting Devices Including Quantum ConfinedSemiconductor Nanoparticles, An Optical Component For A Solid StateLight Device, And Methods”, each of the foregoing being herebyincorporated herein by reference in its entirety.

It will be understood that when an element or layer is referred to asbeing “over” another element or layer, the element or layer can bedirectly on or connected to another element or layer or there can beintervening elements or layers. As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items.

As used herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises” and/or “comprising,” or “includes”and/or “including” when used in this specification, specify the presenceof stated features, regions, integers, steps, operations, elementsand/or components, but do not preclude the presence or addition of oneor more other features, regions, integers, steps, operations, elements,components and/or groups thereof.

It will be understood that, although the terms first, second, third,etc., can be used herein to describe various elements, components,regions, layers and/or sections, these elements, components, regions,layers and/or sections should not be limited by these terms. These termsare only used to distinguish one element, component, region, layer, orsection from another element, component, region, layer, or section.Thus, a first element, component, region, layer, or section discussedbelow could be termed a second element, component, region, layer, orsection without departing from the teachings of the exemplaryembodiments of the invention.

The entire contents of all patent publications and other publicationscited in this disclosure are hereby incorporated herein by reference intheir entirety. Further, when an amount, concentration, or other valueor parameter is given as either a range, preferred range, or a list ofupper preferable values and lower preferable values, this is to beunderstood as specifically disclosing all ranges formed from any pair ofany upper range limit or preferred value and any lower range limit orpreferred value, regardless of whether ranges are separately disclosed.Where a range of numerical values is recited herein, unless otherwisestated, the range is intended to include the endpoints thereof, and allintegers and fractions within the range. It is not intended that thescope of the invention be limited to the specific values recited whendefining a range.

Other embodiments of the present invention will be apparent to thoseskilled in the art from consideration of the present specification andpractice of the present invention disclosed herein. It is intended thatthe present specification and examples be considered as exemplary onlywith a true scope and spirit of the invention being indicated by thefollowing claims and equivalents thereof.

1. A light emitting device comprising: a pair of electrodes; two or morelight emitting elements disposed between the electrodes in a stackedarrangement, wherein a light emitting element comprises a layercomprising an emissive material; and a charge generation elementdisposed between adjacent light emitting elements in the stackedarrangement, the charge generation element comprising a first layercomprising an inorganic n-type semiconductor material and a second layercomprising a hole injection material.
 2. A light emitting device inaccordance with claim 1 wherein the light emitting device isencapsulated.
 3. A light emitting device in accordance with claim 1wherein at least one light emitting element includes an emissivematerial comprising quantum dots.
 4. A light emitting device inaccordance with claim 1 wherein at least one light emitting elementincludes an emissive material comprising quantum dots with amonodisperse size distribution.
 5. A light emitting device in accordancewith claim 4 wherein at least one of the other light emitting elementsincludes an emissive material comprising an organic electroluminescentmaterial.
 6. A light emitting device in accordance with claim 1 whereineach of the light emitting elements includes an emissive materialcomprising quantum dots.
 7. A light emitting device in accordance withclaim 1 wherein each of the light emitting elements includes an emissivematerial comprising quantum dots with a monodisperse size distribution.8. A light emitting device in accordance with claim 1 wherein theinorganic n-type semiconductor material comprises a Group II-VIcompound, a Group III-V compound, a Group IV-VI compound, a GroupI-III-VI compound, a Group II-IV-VI compound, a Group II-IV-V compound,a Group IV element, an alloy including any of the foregoing and/or amixture including any of the foregoing.
 9. A light emitting device inaccordance with claim 1 wherein the inorganic n-type semiconductormaterial comprises nanoparticles of the inorganic n-type semiconductormaterial.
 10. A light emitting device in accordance with claim 1 whereinthe inorganic n-type semiconductor material comprises non-light emittingnanoparticles of the inorganic n-type semiconductor material.
 11. Alight emitting device in accordance with claim 1 wherein the inorganicn-type semiconductor material comprises nanoparticles of the inorganicn-type semiconductor material, wherein the nanoparticles are not surfacepassivated.
 12. A light emitting device in accordance with claim 1wherein the inorganic n-type semiconductor material comprises n-typezinc oxide nanoparticles.
 13. A light emitting device in accordance withclaim 1 wherein the hole injection material comprises an organicmaterial without an inorganic dopant.
 14. A light emitting device inaccordance with claim 1 wherein the first layer comprising an inorganicn-type semiconductor material injects electrons into the contiguouslight emitting element and the second layer comprising a hole injectionmaterial inject holes into the contiguous light emitting element.
 15. Alight emitting device in accordance with claim 1 wherein a lightemitting element further comprises one or more additional layers.
 16. Alight emitting device in accordance with claim 1 wherein a lightemitting element comprises: a first layer comprising a charge transportmaterial; and a second layer comprising the emissive layer.
 17. A lightemitting device in accordance with claim 16 wherein the light emittingelement further comprises a third layer comprising a second chargetransport material on a side of the second layer opposite the firstlayer.
 18. (canceled)
 19. A light emitting device in accordance withclaim 1 wherein the charge generation element is at least 80%transparent to light passage.
 20. A light emitting device in accordancewith claim 3 wherein the emissive layer comprising quantum dots iscontiguous to the first layer of the charge generation element.
 21. Alight emitting device in accordance with claim 3 wherein the lightemitting element contiguous to the second layer of the charge generationelement includes a layer comprising a hole transport material betweenthe emissive layer and the second layer. 22-29. (canceled)